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0709PD: Measure fiber digestibility to improve nutrition

John Goeser Published on 24 April 2009

Feed cost evaluation is an important part of managing your dairy business, especially in these economic times of low milk prices and high input costs. Of these input costs, feed expenditures represent the single greatest expense, accounting for approximately 45 percent of the total cost of milk production.

In addition to critically evaluating your feed costs, you should also evaluate one aspect of your nutrition program you can control throughout the year – forage quality. Forage quality can have the biggest impact on milk production, efficiency and profitability because higher-quality forages allow for more forage to be fed. Since forages typically cost less per pound of dry matter than grains and supplements, more forage could lead to less in feed expenditures.

Fiber content is the biggest variable in determining your forage quality. High-fiber forage (greater than 45 percent NDF) will almost always have less energy per pound than a forage with less fiber (less than 40 percent NDF). This is because fiber digestibility (NDFd, percent of NDF) is lower on average than protein, fat and sugar digestibility. Therefore, a forage with higher protein, fat and sugar content has more energy available per pound than one with less protein, fat and sugar (and more fiber).

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Fiber digestibility is also important to determine your forage’s quality since NDFd can range from 30 to over 70 percent. This range is much greater than that for protein, fat or sugar digestibility. As a result, the digestible energy from fiber ranges widely and NDFd complicates balancing for certain levels of ration NDF from forage (e.g., 19 to 23 percent). NDFd is the reason why current cuttings of alfalfa may not produce as much milk as previous cuttings from prior years, despite both forages being similar in protein and fiber contents.

Forage NDFd relates to rumen fill. Poorly digestible forages limit milk production because the rumen fills with fiber. Rumen fill negatively affects dry matter intake (DMI), and as intake slows, less energy is available for milk production. Forages with greater NDFd will be digested quickly and will not fill the rumen as much as poor-quality forage. In fact, a one-unit improvement in NDFd may lead to an approximately 0.4-pound increase in DMI. Highly digestible forages allow your nutritionist to increase ration forage levels without being in danger of limiting DMI.

Increasing ration forage levels from 50 percent forage to 65 percent forage using high-quality forages can significantly decrease purchased feed costs – potentially $0.90 per cow per day or more. Cost savings are possible because your nutritionist can replace higher-cost grains or byproducts with forages and still meet energy demands. To make these changes and not sacrifice milk production, fiber content and digestibility must be known. However, precisely measuring forage digestibility has been difficult over the past five to 10 years.

The laboratory NDFd techniques used over the past five to 10 years are based on rumen fluid and are called “in vitro” measurements. Each lab offering these wet chemistry measures collects fluid from different cows or steers. This is an issue because ruminal microbial populations can vary greatly from cow to cow, leading to differences in rumen fluid digestive activity.

The lack of consistency in rumen fluid activity causes poor precision with in vitro digestion measures. This is the main reason that your nutritionist should not compare digestibility values from different laboratories or even from the same laboratory over different months of the year. Despite the limitations described above, forages analyzed by wet chemistry in the same lab run (with the same batch of rumen fluid) can be compared and traditional NDFd measurements are valuable for your dairy’s nutrition program.

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Use NDFd in the following situations to help your nutrition program:

1. Switching forages or crops? Be proactive and make ration adjustments using forage NDFd measures instead of reacting to changes in milk production one or two weeks after new forage is fed. Send both samples of your current and new forage to your forage analysis lab at the same time. Request wet chemistry analysis and ask that both samples be analyzed in the same run. This is important! The difference between these samples is accurate.

2. Questionable forage quality? Compare NDFd of your forage and a high-quality forage (for example, your neighbor’s high-quality forage). Send both samples in to the lab at the same time and ask that the samples be analyzed in the same run. The difference between the neighbor’s high-quality forage and yours can help answer your question.

There are also several new, more reliable NDFd tools now available. The new digestibilities are based on standardized rumen fluid and may offer improved precision compared with traditional measures. Because of the improvement in precision, your nutritionist can use these NDFd tools in the following ways (in addition to those mentioned previously):

3. Benchmark high-quality forage! Keep your high-quality forage analysis results and use it as a benchmark to compare with future forages. In the past, you may not have been able to compare your current forage digestibility results to older analyses. However with the new fiber digestion techniques, you can compare these samples with more confidence.

4. Use NDFd as a decision-making tool when considering first-cutting dates. Choose to cut your alfalfa earlier or later based on fiber content and digestibility. Consider cutting alfalfa at less than 40 percent NDF and greater than 50 percent NDFd.

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5. Monitor ration forage NDFd over time and relate forage changes to differences in milk production. You may find better relationships between forage digestibility and milk production over time with this new measure because of the improved precision.

6. Fiber digestibility curves? The 24-, 30- and 48-hour NIR values can be used together in troubleshooting situations. Your analysis laboratory or nutritionist can use three NDFd time points and draw an NDFd curve when more information is critical. A single measurement, for example a 30-hour value, may not adequately represent the full range of digestibility over time. These new tools have been available for several months, and I’ve observed wide differences in the shape of fiber digestion curves in addition to the 30 to 70 percent range mentioned earlier.

A few notes of caution when using any NDFd value. Do not compare values from different forage labs or NDFd methods, and do not use them as real measures of rumen digestion. These tools are indexes, allowing nutritionists to rank forages and adjust rations based on changes in forage digestibility. Forage NDFd measures are not and will never be as precise as crude protein or NDF measures. Lastly, low-fiber forages are almost always higher quality. For example, a forage with 44 percent NDF and 50 percent NDFd may contain 0.65 NEL (Mcal/lb), but a forage with 35 percent NDF and 40 percent NDFd will be higher in energy and may contain 0.68 NEL (Mcal/lb).

By striving for high NDFd in addition to lower fiber content, your dairy can produce high-quality forages. With exceptional forage quality, your nutritionist can push ration forage levels beyond typical ranges and reduce feed costs by replacing higher-cost ingredients, such as corn or soybean meal, with forages. PD

References omitted but are available upon request at

John Goeser
Nutritionist
Vita Plus Corp.

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